The keypad is often employed in modern control systems as a device for entering data or commands into the control electronics. The ubiquitous telephone-style keypad is one example. Most keypad-based control systems feature a symbolic display of alphanumerics which identifies the particular function of each key. Prior art commonly includes the alphanumerics as printed or inscribed characters on the face of each key. Such an approach limits each key to an unchanging, permanent set of characters which are thereby dedicated to their respective keys.
The limitations of the keypad are tested in modern control systems which depend upon a wealth of commands and offer many instances for data entry. Attempts to expand the number and variety of symbols associated per key have featured devices such as replacement templates and adhesive-backed labels to delineate groups of alphanumerics for each key. Such approaches continue to be inflexible, cumbersome, and not under the control of the system addressed. They do not allow the system or the user to reconfigure the symbology according to the requirements of the system operation as it changes mode. Annunciation systems based on cathode-ray or flat-panel displays are expensive and bulky attempts to allow the system to reconfigure the symbolics associated with each key by drawing from a memory cache of possible commands and data characters. However, the complexity of such combinations removes the keypad from the realm of modular, inexpensive, compact, and reliable data-entry devices. Prior art apparatus often places the symbols not on the face of the key, but elsewhere, which detracts from the simplicity and the ergonometric quality of the keypad.